quinta-feira, 22 de setembro de 2016

Can a blanket violate the second law of thermodynamics?

One of the silliest arguments of climate deniers goes like this: the
atmosphere with its greenhouse gases cannot warm the Earth’s surface,
because it is colder than the surface. But heat always flows from warm
to cold and never vice versa, as stated in the second law of
thermodynamics.
The freshly baked Australian Senator Malcolm Roberts has recently phrased it thus in his maiden speech:

"It
is basic. The sun warms the earth’s surface. The surface, by contact,
warms the moving, circulating atmosphere. That means the atmosphere
cools the surface. How then can the atmosphere warm it? It cannot. That
is why their computer models are wrong."

This is of course not only
questions the increasing human-caused greenhouse effect, but in general
our understanding of temperatures on all planets, which goes back to Joseph Fourier, who in 1824 was the first to understand the importance of the greenhouse effect.

The
atmosphere acts like a blanket which inhibits heat loss. In fact
according to Roberts’ logic, a blanket could also not have a warming
effect:

"It’s simple. The body warms the blanket. This means that the blanket cools the body. So how can the blanket warm it? It cannot!"

The
answer is simple. The warm body loses heat to the cold air. The blanket
inhibits and slows this heat loss. Therefore you stay warmer under a
blanket.

The Earth loses heat to the cold universe. The atmosphere
inhibits this heat loss. Therefore, the surface remains warmer than it
would be without the atmosphere.

It is true that the surface loses
heat to the atmosphere – but less than it would otherwise lose directly
to space. Just as I lose less heat to the blanket than I would
otherwise lose to the air, without blanket.

Of course, in neither
case is the second law of thermodynamics violated. The heat always flows
from warm to cold – just more or less effectively. The processes of
heat transfer are quite different – for the blanket it is mainly heat
conduction, for the greenhouse effect it is thermal radiation. The
climate deniers claim that the colder atmosphere cannot radiate thermal
radiation towards the warmer surface. This is of course nonsense. The
cool Earth also sends thermal radiation towards the hot sun – how would
thermal radiation leaving Earth know how warm the surface is that it’s
going to hit? It’s just that the sun sends more radiation back to us –
the net flow is from hot to cold. More is not implied by the second law of thermodynamics.

Thanks
to two Germans (Gerlich and Tscheuschner of the TU Braunscheig – deeply
embarrassing for this university), the absurd claim that the greenhouse
effect violates the second law of thermodynamics even made it into an
obscure physics journal – obviously there was no peer review to speak of. The bizarre article was promptly demolished by some US physicists. Just recently I read the claim again in an article of coal lobbyist Lars Schernikau – with such fairy-tale beliefs of its representatives, one is not surprised by the decline of the coal industry.

The
thermal radiation from the atmosphere toward the ground, which
allegedly cannot exist, is of course routinely measured, including its
increase (see e.g. Philipona et al. 2004, 2012).

And you can even
feel it. Those who sometimes sit outside in the garden after dark know
this. Under a dense, low cloud layer you do not nearly get cold as fast
as on a clear starry night. This is due to the thermal radiation coming
from the clouds. They are colder than our body, but warmer than the
night sky in clear air.

Roberts said: “Like Socrates, I love
asking questions to get to the truth.” Perhaps he will ponder my answer
next time he sits in his garden at night, or slips under a blanket.

P.S.
Here is the energy balance diagram for our Earth, explained in IPCC FAQ 1.1.
The “Back Radiation” makes the greenhouse effect. It is larger than the
solar radiation reaching the ground, and measured by a global radiation
measurement network.